Levée en masse – the spontaneous uprising of the civilian population against an invading force – has long been a part of the modern law of armed conflict with regards to determining who may legitimately participate in armed conflict. The concept originated during the revolutionary wars in America and France, and was incorporated into the first codified rules of armed conflict. However, despite the prevalence of the category of levée en masse in the modern laws of armed conflict, there have been few, if any, instances of levée en masse taking place in modern armed conflicts. This article examines how and why the category of levée en masse developed. In doing so, this paper situates the concept and evolution of levée en masse within the history of international humanitarian law more generally.